1. Field of the Invention
This invention relates to a process for manufacture of an optical disc (OD) master on which data is recorded and from which an OD stamper may be made by conventional electroforming means for mass duplication of many OD's identical to the master, and which process may be alternatively used to make a Write Once Read Many (WORM) OD for test and validation of systems for backup archieval of data, or for general production of recorded OD's.
2. Description of the Prior Art
An OD master which may also include a compact disc (CD) master, is an article from which essentially all prerecorded mass produced consumer OD's are subsequently molded. Indeed, the OD master is a master mold for a stamper which produces many OD's. The master is the first article in the OD manufacturing process that presents data or information on a disc or platter that one can read by optical means and from which copies may be made. All subsequent OD manufacturing and duplication processes or operations transfer that identical form, format and data from one article or disc to the next by various means, to include vacuum deposition, electroforming/plating, or injection molding processes. Prior to making the OD master, data or information would conventionally be recorded on tape or other storage devices, from which the data is transformed to the OD master by optical means for mass duplication thereof.
Past techniques for producing a master combined existing technologies from various industries. The most prominent of these techniques is the photo resist concept of the semiconductor industry. The master, and subsequently molded and duplicated OD's, comprise a series of pits describing optically recorded information that lay on an actual or imagined spiral line or track that starts at an inner radius near the center of the disc and continues to an outer radius near the periphery of the disc. Conventionally, the pits are approximately 0.6 um wide, 0.9 to 3.3 um long, 0.12 um deep, and are separated from neighboring tracks on either side thereof by 1.6 um (10.sup.-6 meters).
To accomplish the above format, the photoresist technique commonly used in the semiconductor industry was used. By such method, a lamina of photoresist material is traditionally spin coated upon a very smooth, polished circular plate of glass. The glass plate with photoresist lamina is then placed in an oven to cure the photo resist. The photoresist coated, glass plate is next set upon a turntable which is caused to spin at a variably decreasing rate of speed to yield a constant linear velocity (CLV) of approximately 1.2 meters per sec for a point on the surface of the glass plate traveling radially outward from the center of the plate. In such systems a data modulated laser is set immediately above the glass plate and is suspended by a relatively large and complex translational mechanism to cause the laser to very gradually, and at a steady radial rate, move outwardly from the center of the spinning glass plate while concomitantly radiating the surface of the plate which in effect produces a spiral track of data in the photo resist material. This process is referred to as mastering. What happens is that the areas designated to become pits are exposed to laser light along the spiral path as the plate rotates. After all data is recorded in the photoresist lamina on the glass plate, the plate is placed in at least one bath of developing solution, typically a multistep process, where the exposed areas in the photoresist are etched out leaving a series of pits. The glass plate with etched photoresist is then oven dried, followed by metalization of the photoresist lamina by vacuum deposition of a thin conductive lamina of metal thereon, which yields the desired master. The master is finally subjected to an analysis and validation process prior to electroforming to make a stamper and replication.
The foregoing type of mastering system is quite expensive in terms of capital equipment involved, labor, space, sensitivity to motion and shock, time and costs. Operating costs to produce a single master can be relatively expensive.
As an alternative to the photoresist mastering system, another technique evolved which incorporates the use of a material which undergoes ablation when exposed to a laser thereby forming the desired pit. This process brought forth a number of advantages over the photoresist process, including a reduction in the process steps by eliminating, among others, developing and curing steps, which results in a less costly procedure and much shorter completion time, and the ability to monitor the quality of the disc concurrent with the mastering process of recording data thereon.
This mastering process, known as non-photo resist (NPR), utilizes what is referred to as a dry process formula (DPF) photoreactive coating. In this case, a unique material which needs no oven-curing, developing or drying procedures, is spin coated upon a glass plate disc. The plate and DPF material are then set upon a turntable for mastering and concomitant analysis. The laser creates pits in real time by ablation permitting simultaneous direct read after write (DRAW) for analysis of error rate and conformance to standards. Metalization of the pitted layer is conducted to complete the mastering process. Once such a master is made, conventional electroplating and replication processes are accomplished to make a stamper which is then used for mass duplication of OD's by conventional injection molding processes.
The NPR system makes some improvement over the prior art largely due to eliminating etching processing and oven-curing but it does not eliminate motion sensitivity nor the need for expensive and complex translator mechanisms and the glass reconditioning processes. Capital expense for such systems is still substantial, and the cost to produce a single master is still relatively expensive.
Yet another mastering process is referred to as a direct metal mastering (DMM) process. In the DMM process, instead of using a laser to either expose or cause ablation of the surface material, a tiny diamond stylus actually engages the surface, usually metal, and gouges the pits. Although technically feasible, this concept has had no meaningful application in industry if for no other reason because the "no contact" laser systems offer inherent increased reliability, longevity and efficiency over mechanical systems.
Therefore, although satisfactory for intended purposes at the time, the prior art possesses inherent limitations, and there continues to be a long standing need for a more efficient, less time consuming, and less expensive method for manufacture of an OD master. The invention disclosed herein does just that.